Okay, did more testing and I find few interesting facts (well, at least it was interesting to me!).
In my previous post, I used unupgraded purify so the counter is +1. Now I'm using the upgraded purify (+2 purify counter)
1. I managed to play 3 early purify before the dune scorpion appear (finally!). I buffed the scorpion with momentum. End turn.

2. The purify counter goes down to +5 and I was inflicted with neurotoxin.

3. In the same turn, I played 5 cards reducing the purify counter to zero - but the game didn't show any counter at all unlike the previous scenario.

(Interesting fact #1)

4. Still in the same turn, I played the 5th card - no neurotoxin counter showed. This suggest that I was somehow free of neurotoxin without ever playing a new purify.

(Interesting fact #2)

5. In the next turn, obviously the scorpion can hit me due to momentum. But the moment the scorpion hit me, the neurotoxin counter went straight to +2.

(Interesting fact #3)

6. I didn't played any cards so the subsequent scorpion's attack increased the neurotoxin counter to +3.

7. I wanted to test further with a new purify in my hand but the stupid AI ate his own scorpion...

I've done few more testing and the same thing happened - including the stupid AI ate his own scorpion, but that's not the point.
- #3 might just be a visual bug, but doesn't it contradict what happen in my previous testing?
- #4 looks like a bug that might caused by #3 because neurotoxin should be still in effect
- #5 is definitely a bug because I can't find the reason why the neurotoxin counter suddenly goes up to +2. And I don't quite understand how my HP goes from 84 to 82.
Appreciate if someone else could provide feedback and/or do more testing to confirm my suspicion.